Starvation may change autonomic nervous system activity and sensitivity such that a greater vagal withdrawal may occur during a sympathetic challenge. Six healthy humans endured a 3-day, water-only fast, during which participants were subjected to passive 80 degrees head-up tilt testing twice on each day (a.m. and p.m.). Heart rate, heart rate variability (HRV), ventilation [Formula: see text], and respiration ([Formula: see text] and [Formula: see text]) were recorded during supine rest and head-up tilting. On Day 1 (a.m.), supine heart rate was 46.0 +/- 3.3 beats min(-1), increasing to 51.6 +/- 7.4 beats min(-1) on Day 3 (p.m.). On Day 1 (a.m.), supine high frequency HRV was 57.9 +/- 31.6(NU), increasing to 69.5 +/- 21.3(NU) on Day 3 (p.m.). Tilt-induced increases in heart rate were greater following starvation (10.5 +/- 7.8 vs. 16.1 +/- 8.6 beats min(-1)), and tilt-induced decreases in high frequency HRV were greater following starvation (-4.1 +/- 27.7 vs. -28.0 +/- 20.8(NU)). Supine V'CO(2) remained unchanged, whereas V'O(2) increased and respiratory exchange ratio decreased (0.91 +/- 0.10 vs. 0.80 +/- 0.05). Greater vagal withdrawal and elevated heart rate induced by head-up tilting during starvation may indicate increased autonomic sensitivity.